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Cold seep ecosystems can support enormous biomasses of free-living and symbiotic chemoautotrophic organisms that get their energy from the oxidation of methane or sulfide. Most of this biomass derives from animals that are associated with bacterial symbionts, which are able to metabolize the chemical resources provided by the seeping fluids. Often these systems also harbor dense accumulations of non-symbiotic megafauna, which can be relevant in exporting chemosynthetically fixed carbon from seeps to the surrounding deep sea. Here we investigated the carbon sources of lithodid crabs (Paralomis sp.) feeding on thiotrophic bacterial mats at an active mud volcano at the Costa Rica subduction zone. To evaluate the dietary carbon source of the crabs, we compared the microbial community in stomach contents with surface sediments covered by microbial mats. The stomach content analyses revealed a dominance of epsilonproteobacterial 16S rRNA gene sequences related to the free-living and epibiotic sulfur oxidiser Sulfurovum sp. We also found Sulfurovum sp. as well as members of the genera Arcobacter and Sulfurimonas in mat-covered surface sediments where Epsilonproteobacteria were highly abundant constituting 10% of total cells. Furthermore, we detected substantial amounts of bacterial fatty acids such as i-C15∶0 and C17∶1ω6c with stable carbon isotope compositions as low as -53‰ in the stomach and muscle tissue. These results indicate that the white microbial mats at Mound 12 are comprised of Epsilonproteobacteria and that microbial mat-derived carbon provides an important contribution to the crab's nutrition. In addition, our lipid analyses also suggest that the crabs feed on other (13)C-depleted organic matter sources, possibly symbiotic megafauna as well as on photosynthetic carbon sources such as sedimentary detritus.

Kessler et al. (Reports, 21 January 2011, p. 312) reported that methane released from the 2010 Deepwater Horizon blowout, approximately 40% of the total hydrocarbon discharge, was consumed quantitatively by methanotrophic bacteria in Gulf of Mexico deep waters over a 4-month period. We find the evidence explicitly linking observed oxygen anomalies to methane consumption ambiguous and extension of these observations to hydrate-derived methane climate forcing premature.

Several methanogenic pathways in oxic surface waters were recently discovered, but their relevance in the natural environment is still unknown. Our study examines distinct methane (CH4) enrichments that repeatedly occur below the thermocline during the summer months in the central Baltic Sea. In agreement with previous studies in this region, we discovered differences in the methane distributions between the western and eastern Gotland Basin, pointing to in situ methane production below the thermocline in the latter (concentration of CH414.1±6.1 nM, δ13C CH4 -62.9 ‰). Through the use of a high-resolution hydrographic model of the Baltic Sea, we showed that methane below the thermocline can be transported by upwelling events towards the sea surface, thus contributing to the methane flux at the sea–air interface. To quantify zooplankton-associated methane production rates, we developed a sea-going methane stripping-oxidation line to determine methane release rates from copepods grazing on 14C -labelled phytoplankton. We found that (1) methane production increased with the number of copepods, (2) higher methane production rates were measured in incubations with Temora longicornis (125±49 fmol methane copepod -1 d -1) than in incubations with Acartia spp. (84±19 fmol CH4 copepod -1 d -1) dominated zooplankton communities, and (3) methane was only produced on a Rhodomonas sp. diet, and not on a cyanobacteria diet. Furthermore, copepod-specific methane production rates increased with incubation time. The latter finding suggests that methanogenic substrates for water-dwelling microbes are released by cell disruption during feeding, defecation, or diffusion from fecal pellets. In the field, particularly high methane concentrations coincided with stations showing a high abundance of DMSP/DMSO-rich Dinophyceae. Lipid biomarkers extracted from phytoplankton- and copepod-rich samples revealed that Dinophyceae are a major food source of the T. longicornis dominated zooplankton community, supporting the proposed link between copepod grazing, DMSP/DMSO release, and the build-up of subthermocline methane enrichments in the central Baltic Sea. [ABSTRACT FROM AUTHOR]

Abstract: We report on methane enrichments that were observed during summer in the upper water column of the Gotland Basin, central Baltic Sea. In the eastern part of the basin, methane concentrations just below the thermocline varied between 15 nM and 77 nM, in contrast to the western part where no methane enrichments could be detected. Stable carbon isotope ratios of methane (δ13C‐CH4 of −67.6‰) indicated its in situ biogenic origin from CO2 reduction, which was supported by clonal sequences that clustered with Methanomicrobiaceae, a family of methanogenic Archaea. Incubation experiments with a Temora longicornis dominated seston fraction obtained from the relevant depth showed a positive correlation between seston concentration and methane production rates. Our results, in combination with previous literature outcomes, suggest that the methane enrichment in the eastern basin might be sustained by a diet‐consumer relationship between the dinoflagellate Dinophysis norvegica and the copepod T. longicornis. However, our mass balance indicates that a local methane production of 110 pmol L−1 d−1 was needed to maintain the methane enrichment, and that the estimated production rate from our incubation experiments of 0.3 pmol CH4 d−1 per adult T. longicornis (about 1 pmol L−1 d−1) was too low to maintain the methane enrichment by zooplankton associated methane production only. These calculations also showed that methane was consumed below the thermocline and not transported into the upper‐ocean, suggesting that other sources in the mixed layer in the range of 95 pmol L−1 d−1 are needed to maintain the observed methane air–sea flux. [ABSTRACT FROM AUTHOR]

Large quantities of methane are stored in hydrates and permafrost within shallow marine sediments in the Arctic Ocean. These reservoirs are highly sensitive to climate warming, but the fate of methane released from sediments is uncertain. Here, we review the principal physical and biogeochemical processes that regulate methane fluxes across the seabed, the fate of this methane in the water column, and potential for its release to the atmosphere. We find that, at present, fluxes of dissolved methane are significantly moderated by anaerobic and aerobic oxidation of methane. If methane fluxes increase then a greater proportion of methane will be transported by advection or in the gas phase, which reduces the efficiency of the methanotrophic sink. Higher freshwater discharge to Arctic shelf seas may increase stratification and inhibit transfer of methane gas to surface waters, although there is some evidence that increased stratification may lead to warming of sub-pycnocline waters, increasing the potential for hydrate dissociation. Loss of sea-ice is likely to increase wind speeds and sea-air exchange of methane will consequently increase. Studies of the distribution and cycling of methane beneath and within sea ice are limited, but it seems likely that the sea-air methane flux is higher during melting in seasonally ice-covered regions. Our review reveals that increased observations around especially the anaerobic and aerobic oxidation of methane, bubble transport, and the effects of ice cover, are required to fully understand the linkages and feedback pathways between climate warming and release of methane from marine sediments. [ABSTRACT FROM AUTHOR]

Abstract: The release of methane from the seafloor into the water column within the hydrate stability field (HSF) is a natural and widely observed process. The subsequent bubble dissolution rate determines how far upwards gas is transported and the vertical distribution of this methane source to the water column. Understanding this process is essential to describing natural deep sea seeps, to assess the hazard potential from blowouts in offshore drilling activities for gas and oil, and to refine past and future scenarios of global change involving large-scale destabilization of gas hydrates and free methane gas. We report on in situ experiments on single methane and argon bubbles within and above the HSF for depths from 400 to 1500 m. Single bubbles were injected from the ROV Ventana into an attached, back-illuminated, flow-through imaging box. The ascent of individual bubbles within the imaging box was recorded and analyzed for rise velocity, V B , and radius shrinkage rate, dr/dt. For all bubbles V B =~30 cms−1. Methane bubbles released within the HSF had markedly enhanced lifetimes, attributed to hydrate skin formation. dr/dt varied from −7.5 μm s−1 above the HSF to –1 μm s−1 at 1500 m, well within the HSF. Bubble longevity within the HSF increased with distance into the P/T-space from the hydrate phase boundary. There was a delay prior to the slower dissolution, where dr/dt for methane bubbles was comparable to dr/dt above the HSF, which was interpreted as a time delay before the onset of hydrate skin formation. Although variable, the onset time generally decreased with distance from the hydrate phase boundary. No delay was observed for the deepest releases. We relate these findings to formal calculations of methane solubility and density as a function of pressure. Pressure-dependent deviations from ideal gas law and Henry''s law were implemented in a numerical bubble-propagation model which incorporated the effects of decreased solubility and surface mobility after hydrate formation. Inclusion of these effects greatly improved model prediction of observed methane bubble behavior within the HSF. The effect of these individual depth-related effects on bubble dissolution rate and longevity then was assessed quantitatively, and assumptions of different gas solubility prior and after hydrate nucleation or an effect of hydrates at the interface on the hydrodynamics at the surface were tested. Here, our approach implements our current knowledge on physicochemical and hydrodynamic control and does not seek the best fit for the given data sets, thus it also reveals current uncertainties in methane bubble processes in the HSF. [Copyright &y& Elsevier]

To constrain the fluxes of methane (CH4) in the water colunm above the accretionary wedge along the Cascadia continental margin, we measured methane and its stable carbon isotope signature (δ13C-CH4). The studies focused on Hydrate Ridge (HR), where venting occurs in the presence of gas-hydrate-bearing sediments. The vent CH4 has a light δ13C-CH4 biogenic signature (-63 to -66‰ PDB) and forms thin zones of elevated methane concentrations several tens of meters above the ocean floor in the overlying water column. These concentrations, ranging up to 4400 nmol L-1, vary by 3 orders of magnitude over periods of only a few hours. The poleward undercurrent of the California Current system rapidly dilutes the vent methane and distributes it widely within the gas hydrate stability zone (GHSZ). Above 480 m water depth, the methane budget is dominated by isotopically heavier CH4 from the shelf and upper slope, where mixtures of various local biogenic and thennogenic methane sources were detected (-56 to -28‰ PDB). The distribution of dissolved methane in the working area can be represented by mixtures of methane from the two primary source regions with an isotopically heavy background component (-25 to -6‰ PDB). Methane oxidation rates of 0.09 to 4.1‰ per day are small in comparison to the timescales of advection. This highly variable physical regime precludes a simple characterization and tracing of "downcurrent" plumes. However, methane inventories and current measurements suggest a methane flux of approximately 3 × 104 mol h-1 for the working area (1230 km2), and this is dominated by the shallower sources. We estimate that the combined vent sites on HR produce 0.6 × l04 mol h-1, and this is primarily released in the gas phase rather than dissolved within fluid seeps. There is no evidence that significant amounts of this methane are released to the atmosphere locally. [ABSTRACT FROM AUTHOR]

To help constrain models involving the chemical stability and lifetime of gas clathrate hydrates exposed at the seafloor, dissolution rates of pure methane and carbon-dioxide hydrates were measured directly on the seafloor within the nominal pressure-temperature (P/T) range of the gas hydrate stability zone. Other natural boundary conditions included variable flow velocity and undersaturation of seawater with respect to the hydrate-forming species. Four cylindrical test specimens of pure, polycrystalline CH4 and CO2 hydrate were grown and fully compacted in the laboratory, then transferred by pressure vessel to the seafloor (1028 m depth), exposed to the deep ocean environment, and monitored for 27 hours using time-lapse and HDTV cameras. Video analysis showed diameter reductions at rates between 0.94 and 1.20 μm/s and between 9.0 and 10.6 · 10−2 μm/s for the CO2 and CH4 hydrates, respectively, corresponding to dissolution rates of 4.15 ± 0.5 mmol CO2/m2s and 0.37 ± 0.03 mmol CH4/m2s. The ratio of the dissolution rates fits a diffusive boundary layer model that incorporates relative gas solubilities appropriate to the field site, which implies that the kinetics of the dissolution of both hydrates is diffusion-controlled. The observed dissolution of several mm (CH4) or tens of mm (CO2) of hydrate from the sample surfaces per day has major implications for estimating the longevity of natural gas hydrate outcrops as well as for the possible roles of CO2 hydrates in marine carbon sequestration strategies. [Copyright &y& Elsevier]

A fully automated measurement system for recording of the surface water CO 2 partial pressure, pCO 2 , was deployed on VOS (voluntary observation ship) “Finnpartner/Finnmaid” in 2003. Since 2009, an amendment of the system also allows for the continuous detection of the surface water partial pressure of methane, pCH 4 . The ship commutes regularly at 2–3 day intervals between the Gulf of Finland (Helsinki) and the Mecklenburg Bight (Lübeck) in the southwest of the Baltic Sea. The pCO 2 data in the central Gotland Sea showed a pronounced seasonality that was mainly controlled by the biological production and decomposition of organic matter in combination with stratification/mixing of the water column. CO 2 consumption in spring/summer caused pCO 2 that were by up to 300 μatm below the atmospheric level. In contrast, the pCO 2 exceeded the atmospheric values during autumn/winter when deep mixing transports CO 2 -enriched water to the surface. To identify the central Baltic Sea as a sink or source for atmospheric CO 2 , an air–sea CO 2 gas exchange balance was established for three selected years (2005, 2008 and 2009). During each year the surface water acted as a net sink for atmospheric CO 2 with uptake rates ranging between 0.60 and 0.89 mol m − 2 yr − 1 . The rates correspond approximately to the enhanced carbon burial in sediments during the last century and suggest a link between eutrophication and CO 2 uptake. The data of the surface methane concentration are used to focus on situations were extraordinarily high methane concentrations were observed. Temporary methane peaks were observed south of the Island of Gotland, which could clearly be attributed to frequent upwelling events. Between spring 2012 and 2013, Finnmaid went at a few occasions to St. Petersburg in the east of the Gulf of Finland. Methane concentration of up to 130 nmol L − 1 where found close to the River Neva mouth but decreased rapidly to the west. The plume of methane-enriched waters was observed farthest to the west during the winter period. This was attributed to air–sea gas exchange that was most effective during summer but inhibited during winter because of the ice coverage. [ABSTRACT FROM AUTHOR]

Both the frequency and intensity of drought events are expected to increase, with unresolved alterations to peatland methane cycling and the involved microbial communities. While existing studies have assessed drought effects via experimental approaches under controlled conditions, to our knowledge, no studies have examined the in-situ effects of natural drought in restored temperate fens. In this study, we used quantitative polymerase chain reaction (qPCR) and high throughput 16S rRNA gene amplicon sequencing of DNA and complementary DNA (cDNA) to determine the abundances and community structure of total and putatively active microbial communities following the 2018 European summer drought. Together with geochemical and methane flux data, we compared these results to a non-drought reference dataset. During drought, water level and methane flux rates decreased to a new recent minimum in both fens. This corresponded with pronounced shifts in porewater geochemistry. Microbial community composition in the drought year differed markedly, and was characterized by a greater relative and total abundance of aerobic methanotrophs, and, in one of the two sites, by a decrease in total methanogen abundance. In contrast to the non-drought reference years, type I methanotrophs were clearly more dominant than type II methanotrophs in both fens. cDNA sequencing confirmed the activity of type I methanotrophs during drought, with Methylomonaceae having the highest average relative abundance of bacterial cDNA transcripts. We show that changes in microbial community dynamics, porewater geochemistry, and ecosystem methane fluxes can be substantial following natural drought in restored fens, and provide the first in-situ evidence from a natural drought which suggests type I methanotroph populations are more active than type II methanotrophs in response to drought effects. Type I methanotrophs may represent a key microbial control over methane emissions in restored temperate fens subject to natural drought. • Drought decreased water level and ecosystem methane fluxes substantially. • Microbial community dynamics reflected drought conditions at the sites. • Methanogen gene abundances decreased in one of the two fens. • Proportion of type II methanotrophs was low and similar to a non-drought year. • Type I methanotrophs dominated the active bacterial community profile at both sites. [ABSTRACT FROM AUTHOR]

Through frequent inundation, erosion and solution processes, sea level rise, which isconsidered to be one of the main impacts of climate change, alters areal extensions of coastalpeatlands. Coastline retreat shifts adjacent ecosystems and results in the submergence ofterrestrial, organic carbon rich peat sediments in the sea. The coastal peatland Heiligenseeand Hütelmoor near Rostock-Warnemünde is a very unique ecosystem due to its strongconnection with the Baltic Sea. Using sediment cores and shallow geo-acoustic surveys, wefound Holocene peat deposits (Corg of 37-53%, 14C-dated to 6725 ± 87 cal yr BP) tocontinue more than 90 m (areal extend: 0.16-0.2 km2) offshore from the recent peatland(Kreuzburg et al., 2018, doi: 10.3389/feart.2018.00103). The location of outcropping peatdeposits coincides with temperature, salinity and bottom water methane anomalies (CH4= 42.4 ± 12.9 nmol L−1) most likely originating from submarine groundwaterdischarge.Motivated by these findings in the field, we investigated the release of peat-derived dissolvedorganic carbon (DOC) and biogeochemical processes that govern carbon transformations by a50-day column experiment, simulating alternating episodes of brackish water (S ∼16)intrusion and seepage of anoxic, low-saline groundwater. Results imply that discharge oflow-saline groundwater through the peat soils significantly increased the release of DOC,with the source confirmed by 13CDOC signatures of -26.9 to -27.7 ‰Ẇe further observed thatthe concentrations of DOC correlate positively with dissolved inorganic carbon(DIC), suggesting rapid organic matter mineralization. In contrast, oxygenatedbrackish water intrusions show a contrary effect with decreasing DOC and DICconcentrations in the pore water and significantly lower CH4 and CO2 emissions.These, to date unmonitored dynamics, reveal a potential source of CO2 and CH4from coastal sediments with submerged peat, and show that the production andrelease of carbon is significantly dependent on the flow regime and on the sedimentcharacteristics. [ABSTRACT FROM AUTHOR]